Automatic tuning device for radio receivers



y 9, 1940- E. N. MULLER 2,207,467

AUTOMATIC TUNING DEVI GE FOR RADIO RECEIVERS Filed June 29,1937 2Sheets-Sheet 1 y 1940. E. N. MULLER 2,287,467

AUTOMATIC TUNING DEVICE FOR RADIO RECEIVERS 7 Filed June 29, 1937 2Sheets-Sheet 2 Patented July 9, 1940 AUTOMATIC TUNING DEVICE FORRECEIVERS FlCE asmo Egon NicolasMuller, Esch, Alzette, Luxemburg 8Claims.

My present invention applies to radio receivers of the type preferablywherein the tuning frequency may be varied through a range ofsubstantial width, and relates more particularly to apparatus, and tothe attendant circuits, for facilitating the tuning-in of the desiredprogram.

In' most receivers due to the high selectivity thereof, some difficultyis experienced in exactly tuning in the incoming signal Of course, whenthe receiver is not sufficiently well in resonance with the carrierfrequency, this will result in a distorted reproduction. It is wellknown, that those receivers wherein the operator must rely on visual oraural indications, are scarcely sat isfactory since a considerableamount of care is required which most listeners are unwilling or unableto expend. Frequently, a so called interstation noise suppressor meanshas been incorporated in modern receivers, to render the. tun ingapparently very sharp whereby to prevent a distorted reproduction.However, the operator in such eventuality might tune past the stations,unless he tunes very carefully, this making the searching of thedifferent programs very tedious. This will readily be understood, whenconsidering that in selective receivers a program can be adequatelyappreciated over a very restricted range of frequencies, not seriouslyin excess of 300 cycles on either side of the exact position, whereasthe spacing of the various stations usually is not less than 10kilocycles, even if it is assumed that there are no idle channels.

Another disadvantage of the conventional noise suppressor constructionsis that they are comparatively expensive, since they should be pure lyelectric in operation to permit a sufiiciently low inertia, and thatthey frequentl may give rise to distortions when the receiver is notstrictly in resonance.

' To avoid some of the above inconveniences, so-

called automatic frequency control systems have been utilized in view ofsubstantially suppressing manual tuning inaccuracies. However, suchdevices are comparatively expensive, and their adjustment and circuitdesign is comparatively very critical when they are to be eifective whensearching the different programs.

Accordingly, .a broad object of the invention consists in the provisionof means for avoiding the above inconveniences and drawbacks ofv theknown systems. More particularly, an object of my invention consists inthe provision of means whereby to spread the tuning positions, by whichthe entertainment value of the program can be adequately appreciated orlistened to without distortions, without thereby increasing the sweep ofthetuning control. member between the stations.

The operator accordingly needs no longer rapidly to brake or arrest thetuning knob as the station is heard, and he may, if desired, uniformlyoperate the tuning member whereby each program will be heard for adesired brief period of time, and quickly thereafter replaced by thefol-- lowing one. V

An advantage of the invention is that the correct operation of a noisesuppressor of simple character may easily be ensured.

The above and further objects which will re-- sult from the detaileddescription, I prefer to attain in accordance with my present invention,by means of electro-mechanical apparatus, which is spectacular in itseffects, and permits attaining any desired proportion of spread of thestation. Apparatus in accordance with'the invention is cheap and easilymanufactured, and may easily be incorporated in the receiver withoutgiving rise to delicate circuit adjustments.

More specifically, according to my present invention, I providevariable-speed mechanism, for normally operating the tuning means at adesired moderate speed, and for increasing the speed of operation in adesired proportion when the receiver is sufficiently in resonance with acarrier wave, by suitably altering the'effective gearratio of theslow-motion drive system for the tuning means.

In the accompanying drawings, Fig. lshows one particular embodiment ofcontrol circuit 3 for supplying the control potentials together withsome of the mechanical elements of an embodiment of variable-speedmechanism, the control potentials in this instance being effective. assoon as the amplitude of substantially any incoming waves which thereceiver is responsive to exceeds a predetermined intensity.

Fig. 2 shows a perspective View of an embodiment of variable-speedmechanism, the speed of which is adapted to be altered stepwisefrom aposition of low drive ratio to a position of high drive ratio, andvice-versa. i

Fig. 3 is a graph. showing change in frequency with respect to operationof the tuning control member, in response to the operation of themechanism in Fig. 2.

Fig. 4 shows a practical construction of the mechanism in Fig, 2.

Fig. 5 shows an alternative form of control circuit wherein the controlpotential is eifective in the'event only of the incoming carrier orother wave exceeding in a desired proportion the background of noise dueto static. The lower right-hand portion of this figure also shows anelevational front view of some of the mechanical elements of a modifiedarrangement of variable-speed mechanism. Fig. 5a shows a sectional viewof the modified tuning mechanism; Fig. 5b is a side-view in elevationthereof; Fig. 5c is a perspective view of this mechanism.

In the accompanying drawings, similar circuit elements or parts areindicated by the same or analogous reference numerals.

Reference should now be had to Fig. 1, wherein the invention is arrangedfor use with a superheterodyne receiver. The signal energy from thecollector I is applied to a tunable radiofrequency amplifier 2, theoutputqof which is coupled to a frequency changer device comprising amodulator stageor first detector 3, which co-operates with a tunablelocal oscillator 4, so as to produce beat oscillations of a fixedintermediate frequency. As is common practice in modern receivers, thedifferent tunable circuits in the sections 2, 3 and 4 areuni-controlled, and in the instance shown each of these receiversections includes a variable tuning-condenser, these condensers beingganged and adapted to be manually operated by means of a tuning knob,which is coupled to the spindle of the gang-condensers through thevariable-speed mechanism described hereinafter.

The intermediate-frequency oscillations are applied to a tunedamplifier-section 5, the output circuit of which is coupled to thedemodulating detector 8 through a transformer with a resonant primarycircuit 6 and a resonant secondary I, both circuits being tuned to theoperating intermediate frequency, and being moderately coupled by mutualinductance.

The second detector is coupled to an audiofrequency amplifier-section 9which serves to actuate a reproducer l6 associated with an outputtransformer II.

The signal energy across the tuned circuit 6 is maintained substantiallyuniform despite variations in the strength of the signals at the inputcircuit of the receiver, by means of an automatic volume control networkof conventional design, which comprises a. rectifier-amplifier [6 fedwith signal-energy from the resonant circuit 6, through the condenser H.The uni-directional potentials which are produced at the output of thedevice l6, are applied to the amplifiers 2 and 5, through the lead I8,whereby to regulate the gain of these receiver sections.

The variable-speed mechanism described in detail hereinafter iscontrolled in response to the output of a control channel which includesa high frequency amplifier tube I9, a rectifier 26 and a direct-currentamplifier tube 29. The input circuit of the amplifier I9 is coupled tothe resonant circuit 6 through a condenser 20, the grid of the tube l9being grounded through the grid leak 2i. The cathode of this tube isslightly positive with respect to ground, whereby the tube amplifiesunder optimum conditions. The rectifier 26 is of the diode type and iscoupled to the plate circuit of the amplifier I9 through a transformerwith a resonant primary 24 and a resonant secondary circuit 25, thesetwo circuits being tuned to the operating intermediate frequency, andbeing loosely coupled, as by mutual conductance. Accordingly, when theincoming signal is substantially in tune, the signal energy iseffectively transferred to the rectifier 26, and same moreover presentsin substance a predetermined amplitude by virtue of the automatic gaincontrol. However the signal when mistuned will be effectivelyattenuated, and will not be able to apply an appreciable proportion ofenergy to the rectifier 26.

The cathode of the diode 26 is connected to ground through aload-resistor 21, whereby a positive D. C. potential will be producedacross cathode and ground in response to the amplitude of the signal asapplied to the rectifier. This potential is applied to the grid of thetube 29 through a filtering network 28 comprising a resistance andcapacity, to prevent the transfer of signal energy to the tube 29. Theanode current of this tube is arranged to fiow through the (main)winding 30 of an electromagnet which forms part of the variable-speedmechanism. This electromagnet comprises a U-shaped core 13 of soft iron,the magnet coil being wound on the leg 13a. The magnet armature 33 ishinged at 34 and is provided with a lever portion 33a, the end of whichcarries a sharp-edged projection Il l adapted in the attracted conditionto engage the rim of a toothed disk 32 which forms an importantconstituent part of the embodiment of control mechanism described indetail hereinafter. The armature is normally maintained a small distancefrom the upper end of the core 1311. by means of the tensioned spring36, whereby to leave some small clearance between the projection H4 andthe teeth of the disk 32, the proportion of clearance being limited bymeans of an abutment l5.

The cathode of the tube 29 is adjusted to a suitable positive potential,whereby the grid is normally biased to a substantial negative potential.Accordingly in the absence of signal energy across the rectifier 26 nocurrent will flow through the magnet-winding 30.

It may be observed that in most instances the conventional automaticgain control networks are not sufficiently effective to maintain astrictly uniform energy across the circuit 6; in actual fact, the levelof energy will be somewhat higher when the carrier tuned in verypowerful. Accordingly, unless the resonant circuits 24, 25 in thecontrol channel are highly selective, thus making the adjustment thereofexcessively critical, a substantial amount of energy will be stances, itis preferable to avoid the above inconvenience by providing acompensation circuit, whereby in substance to balance out, from thepoint of view of the operation of the electromagnet, the amplitudevariations left by an A. V. C.

device the effectiveness of which is comparatively reduced. To this end,the operation of the magnet may be made dependent on the output of asecond rectifier fed through a path with lesser selectivity than thefeeding path of the control potentials are derived from the A. V. C.rectifier, the super-imposition of the two sets of potentials being inthe electro-magnet provided to thisend with a compensating winding 30a,

rectifier 26. In the instance shown, the auxiliary which through theleads 37 is connected in the anode circuit of an amplifier tube in theamplifier section 5, to the grid of which are applied A. V. C.potentials over the tine I0. Since an increased signal strength bringsabout a decreased negative bias of the tube 29 and an increased negativebias of the amplifier 5, both windings should in effect be connected sothat their magnetic fields are in the same direction. The compensatingpotentials are preferably effective only when they exceed a predetermindamplitude, the A. V. C. device being to this end preferably providedwith a delay-bias, arrangements of this character being well-known intheart. In the absence of a signal the flow of current due to the tube 5may be moderate, same depending on the effectiveness of the A. V. C.device.

The circuits in Fig. 1 also comprise a threepole switch 38A38B38C, ofwhich the central contact 38A is grounded, and in one of the switchpositions 380 is connected to the upper end of the loud-speaker circuit,through a bypass condenser I2 of high capacity value, whereby in effectto short-circuit the output of the receiver; whereas in the otherswitch-position SBA-30B the current of the secondary of the mainstransformer I 4 is arranged to flow through the incandescent lamp I5 toilluminate same; this lamp may be arranged behind the tuning dial toilluminate the name of the station which is in tune.

Before I shall proceed to the explanation of the operation of thecontrol circuit in Fig. l,

the remaining portions of the variable-speed mechanism will first bedescribed, with reference to Fig. 2. The mechanism shown is adapted tobe interposed between the tuning control on one hand, and the gangedtuning condensers for setting the tuning frequency of the receiver, onthe other hand. The spindle I00 may to this end carry a knob adapted tobe manually actuated by the operator, in well-known fashion. Whereas forthe convenience of description the tuning drive means in the foregoingare referred to as a tuning knob, which may be directly operatedmanually, it will readily be seen that flexible operating spindles, orfly-wheels, or other devices well-known per se, may serve in connectionwith a device according to the invention, for providing indirect orremote tuning; or again the tuning means may be adjusted by the aid ofan electrical motor, which may be appropriately controlled manually, byswitches. served that in most instances the character of the tuningcontrol unit does not bring about any particular difiiculties, and maybe designed along the lines well-known in the art. The spindle I05 mightbe directly coupled to the spindle of the tuning unit; however when avariable condenser is employed, the entire sweep of which frequentlydoes not exceed half a turn, it is usually preferable to arrange aslow-motion drive between the spindle I05 and the actuating spindle ofthe condenser unit. This slow-motion drive may be of any well-knownconstruction, for instance of the friction type. The value of ratiothereof may be moderate, and it will be observed that the mechanismshown will effectively increase the resulting ratio even when in thelowratio condition. Whilst it may be preferable to select a slow-motiondrive which shall not be hard in operation, it will be appreciated thatthe construction of the mechanism shown would be adequate in practicallyevery instance since capable oftransmitting a torque of high value.

It will be 010-.

There is provided planetary gearing indicated bythe general referencenumber IOI, for combining the effects upon the spindle I05 of twomovements deriving from the rotation of the control spindle I06 throughdifferent ratio values. As shown, the planetary gearing comprises threeelements, viz. "an inner main wheel I02, an outer main wheel I03 and aplanet wheel I04. All of these elements might be cog-wheels, but I havepreferred to show same as being of the friction type.

The inner main wheel IN is pulley-shaped and is coupled to the drivingspindle I06 through clutch mechanism, and is secured to an auxiliaryspindle I061:v in alignment With'the spindle I06. The clutch comprises adriving flange I I2 secured to the spindlel06, a driven clutch member II3 on the spindle I06a and a spider-shaped spring II6 frictionallyengaging the driving and driven parts. It will'be understood that asuitable arrangement of the supporting bearings (not shown) will preventrelative axial displacements of the spindles I00 and IBM andclutch-members thereon. The driven clutch-member H3 is diskshaped andprovided with fine teeth and is adapted to co-operate with theprojection II4 of the arresting lever 35, all as described hereinbeforewith reference to Fig. 1.

The outer main-wheel I03 of. the planetary gearing in effect is shapedas an annulus and is adapted to be driven at all times by the controlspindle I06 through auxiliary slow-motion drive mechanism. The annulusI03 is arranged to be rotatable about the ideal axis of the spindles I00and I06a, and apart from an inner race I03I which is meshing with theplanet Wheel, same comprises an outer race I032 for co-operation withthe auxiliary slow-motion drive. The latter includes a spindle IIO whichis parallel to the main spindles I06-I06a-I05, and is supported bybearings, not shown, in invariable relation with respect to the chassis.The spindle IIO carries a friction disk I08 meshing with a pulley I01firm with the control spindle I06,-and a pulley III meshing with theouter race I032 of the annulus I 03, the disk I08 and pulley III beingin invariable rotational relation. It will be understood that as thespindle I06 is rotated, the annulus I03 is moving with a seriously lowerspeed, as defined by the relative diameters of the elements I0I-I08 andIII-I032. The speed reduction might for instance be 1:40.

The planet wheel I 04 is pulley-shaped, and is positioned freely torotate about the spindle I05a, and is meshing simultaneously with thepulley I02 and with-the race I03I. When driven by the latter elements,the spindle will move along a circle the center of which is on the idealaxis of the spindle I00, this motion being imparted to the spindle I05which is in alignment with the spindle I06, the spindles I05 and I05abeing to this end kept in invariable mechanical relation.

The operations of the system is as follows:

When the receiver is not tuned to a'carrier wave, no potential will beset up across the load resistor 21 of the diode 26, and the tube 29 willbe left excess-biased, whereby no substantial current will fiow throughthe winding 30 of the electro-magnet: On the other hand, there will beno A. V. 0. potential, and the anode current of the amplifier 5 flowingthrough the Winding 30a will be maximum, matters being arranged so thatthis current is insufficient to cause attraction of the magnet armature.Accordingly, by virtue of the friction between the clutch-parts H2; H3,the

pu1ley I02 will be driven with the same speed as the tuning controlspindle I06, whereas the race I03I of the planetary will be driven at aconsiderably lower speed, in the same direction. The mediate pulley I04will accordingly rotate in response to the combined and additive effectsof these two movements, of which the former of course is predominant,and the spindle I05 will be driven with a relatively predetermined speedwhich in the main is defined by the construction of the planetary drive,which operates as a slowmotion drive of moderate ratio.

When the receiver is nearly in tune with a weak station, the proportionof mistuning being less than 300 cycles, a potential of substantiallypredetermined amplitude will be present across 21, the excess-bias ofthe tube 29 being removed whereby a current of considerable amplitudewill flow through the magnet winding 30, which is sufficient to bringabout to attraction of the magnet armature; the intensity of the currentthrough the winding 30a does not however appreciably change, since theamplitude of the station is insufficient to bring about an appreciableA. V. C. potential. The projection II4 by engaging the toothed rim ofthe disk I I3 prevents further rotation of the spindle I06a and pulleyI02 thereon, any further rotation of the control spindle bringing abouta slip between the two clutch-parts. However, since the outer main wheelI03 continues to rotate with a relatively predetermined speed, thepulley I04 and the tuning spindle I05 continue to rotate slowly, wherebythe tuning frequency gradually approaches strict resonance andthereafter brings about gradually increasing proportions of mistuningbeyond the point of exact resonance.

As soon as the receiver is mistuned by about 300 cycles, the potentialacross the diode 26 is insufiicient to cause energization of theelectromagnet. The spring causes retraction of the arresting projectionH4, whereby the direct coupling between the control spindle and thepulley I02 is restored, the tuning frequency of the receiver thusquickly retiring from the resonance frequency of the station which wasin tune.

Obviously, the operator is at liberty at any instant to cease turningthe control spindle, and this he would do at some desired instant whenthe high gear ratio is effective, this condition being indicatedvisually and aurally by the circuit elements which are actuated throughthe switch 30 A-C, the proportion of mistuning in this event being lowerthan some 300 cycles, so as to prevent a distorted reproduction. Itshould be noted however that in certain instances an appreciable higheramount of mistuning would be admissible in which case the device wouldoperate to prevent tuning past the station, even When the control knobis operated very carelessly.

Let it now be assumed that the'receiver is in resonance with a powerfulstation. A- comparatively higher potential will now be effective acrossthe load-resistor 21 of the diode 26, as compared with the foregoinginstance. Accordingly, the negative bias of the tube 29 will be furtherreduced, and the current through the winding 30 will be more intense.However, since a considerable A. V. C. potential is applied to theamplifier 5, the current through the winding 30a will be very small,whereby the sum of the effects of the two windings will in substanceremain constant. In immediate proximity of exact resonance, the currentthrough the winding 30a tends to increase, whereas the current throughthe winding 30m must quickly decrease; however the operation of theelectro-magnet, as a function of the proportion of mistuning does notappreciably suffer since the selectivity of the control circuit isappreciably higher than the selectivity of the A. V. C. channel, wherebythe change in current through 30 will be considerably more marked.

It may be noted that when the control knob is actuated rather speedilyand since the armature of the electro-magnet presentssome inertia, thehigh gear ratio will be rendered effective only when the receiver isatthe point of exact resonance, or even when this point is alreadyexceeded; whereas when the operator tunes more carefully, thechange-over of gear ratio will take place when the proportion ofmistuning is still some 300 cycles. The proportion of spreading of thetuning positions is thus dependable on the mode of actuation of thetuning control, this being advantageous.

The operation of the control device is indicated graphically in Fig. 3,in which the movement of the tuning knob or other control member isplotted horizontally, and the resonance frequency of the receiver isplotted vertically. It will be seen that a comparatively very smallvariation in frequency in immediate proximity of the exact resonanceupon a carrier wave such as FI or F2, corresponds to a comparativelyvery large sweep of the control knob; frequently it is convenient toselect the ratio values of the variable gearing in such a way that thesweep of the control knob for turning from one station to the next maybe about the same width as the spreaded in-tune positions.

In the practical construction shown in Fig. 4, which in substanceconforms to that indicated in more schematic fashion in Fig. 2, theplanetary gearing is of a ball-bearing type, widely used in radioreceivers as a conventional slowmotion drive. The drive ratio thereofmay be 1:5.5, assuming that the outer ball-race is maintainedstationary. The ball-cage I25 is fixed upon the spindle I05, whichlatter is preferably coupled to the gang condensers through anadditional slow-motion drive of moderate ratio. The outer ball-race I23is formed with the circular extension I032 in such a way as to form afriction disk, which engages the pulley III. The inner ball-race I2I issuitably lengthened and provided with a recess which is engaged by theend of the control spindle I06 opposite the tunin knob (not shown). Thetoothed disk H3 is fastened upon the lengthened inner race, and isirictionally engaged by the spider-shaped spring H0 which on the otherhand bears on a flange I ll of the control spindle I06, relative axialmovement being prevented by a suitable arrangement of the bearings, oneof which is shown at H8. The control spindle also carries the pulley I01which is fastened thereon, and which meshes with the friction disk I08,the latter firm with the same auxiliary spindle as the pulley III.

It will be understood that when the control device is operated inresponse to the output of the control circuit in Fig. 1, the sensitivityof the receiver should be adjusted to a suitable valve in response tothe prevailing local reception conditions, there being to this endprovided suitable means suchas a variable biasing potentiometer formanually altering the gain of the amplifier section 5. Of course, whenthe level of back-ground noise due to static happens to be high, thesen- 7 sitivity of the receiver being not reduced in a sufficientproportion, the correct operation of the control mechanism will be upsetsince sufficient energy will be applied to the diode 26 to simulate acarrier wave and to cause attraction of the electromagnet at all times.On the other hand, when the sensitivity of the receiver is-lowered tooconsiderably, the receiver will permit the reception of the very strongstations only. In fact, an inexperienced operator is not able as ageneral rule of adequately making use of his receiver, particularly soas the intensity of the noise is subject to considerable variations andmoreover varies over the different frequency bands.

The above inconvenience of course results from the fact that the controlcircuit is not capable of discriminating between the carrier waves onone hand, and the back-ground of noise due to static on the other handapart from the absolute intensity level of the respective waves, thisinconvenience being besides also present in the many devices at presentknown for providing interstation noise suppression, or illumination ofan indicator lamp when the receiver is in tune with a carrier wave, orfor more or less related purposes. To avoid this drawback, I may preferto utilize a control circuit for detecting the presence and thecondition of exact resonance of a carrier wave, of the type disclosed inthe specification of Luxernburg Patent No. 22,599 of August 1, 1936 (inU. S. application Ser. No. 155,909 filed July 27, 1937). An advantage ofthis device resides in the fact that the sensitivity of the receiver mayat all times be a maximum permissible by the prevailing receptionconditions, whichever may be the skill of the operator. For a completedisclosure of the details of the method, reference should be had to thespecification mentioned; it may be stated however that the operation ofthe control circuit depends on the cooperation of at least tworectifiers, operation of the tuning device or other device foremployment of the control potentials being permitted, only when theintensity of the output of one of these rectifiers associated withselective networks for providing peak response at a frequency orfrequencies somewhat spaced from the intermediate frequency, is below agiven value with respect to the output of the second rectifier havingpeak response in proximity of the intermediate fre quency, so as to showoff the absence of noise in excess of a predetermined level relative tothe level of the desired wave.

Reference should now be had to Fig. 5 which illustrates an embodiment ofcontrol-circuit of the character just indicated, in combination with amodified arrangement of variable-speed gearing, the latter also shown inthe sectional view of Fig. 5a.

The control channel includes a high-frequency amplifier tube 191 theinput circuit of which is coupled to the tuned circuit 6 arranged in amanner similar to that indicated for the tube 19 in Fig. 1. The anodecircuit of the amplifier l9l includes a resonant circuit 4! theselectivity properties of which may be moderate or low, and which istuned to theoperating intermediate frequency. The high-frequency energyacross the circuit 4! is transferred to the rectifiers 46 and 41, whichare indirectly heated diodes. tifier 45 is arranged to produce positivepotentials across its load resistor 49, whereas the rectifier 41 isadapted to set up "negative potentials across the load resistor 48thereof. The load re- .given the same value.

The recsistors 49 and 48 are connected in series,'and the resultingsuper-imposed potential variations with respect to ground aretransferred to-the grid of the amplifier I9! over the line 50. The anodecircuit of this tube also includes the wind: ing 30 of the electromagnetwhich forms part of the gearing mechanism, and which is by-passed fromthe high-frequency point of view, the tube I9l' accordingly performingat the same time the teristics of the receiver portion ahead of thecondenser 42, the peaks of maximum response are left suificientlypronounced for instance'at :15 kilocycles off resonance, with asufiiciently' marked response depression at the operating intermediatefrequency, the difference in the responses being for instance 1:2.5. Theresponse tov the frequencies of the adjacent carrier'waves is preferablykept very low.

The rectifier 41 is coupled to the circuit 4l' through a resonantcircuit 45 tuned to the in"- termediate frequency, the selectivitythereof being, preferably rather sharp, though actual fact thisrequirement is not essential.

The level of energy of true intermediate frequency is arranged to be the'same across both rectifiers. The circuit of the rectifier 46 in cludesa biasing source of potentials, the cathode of the diode 46 being forinstance biased at +2.5 volts with respect to ground, to prevent theappearance of rectified potentials across the load' resistor until thehigh-frequency energy impressed on the rectifier exceeds the thresholdbias. Each load resistor 49, 48 is associated with a filtering network5|, 52, to avoid undesired inter-action between the associated circuits,the

time-constant of each filter being conveniently A filter 53 comprising aresistance and capacity is, also arranged in the line 50 to preventhigh-frequency being fed back on the grid of the amplifier l 9 l.frequency energy is impressed on the diodes 46," 41, the potential ofthe grid at 19! is that of the biasing source of potentials associatedwith the load resistor 49, the cathode potential of the tube being givena slightly higher value so that the grid may be slightly negative withrespect to the cathode, the anode current thus being a max springaction, the armature when attracted caus ing retraction of the arrestingprojection. However, with the modified construction of-variablespeedmechanism more particularly shown in Figs. 5a, 5b, 5c, theelectro-magnet' may be ar- When no highranged in the same way as in Fig.1, as explained more particularly hereinafter.

The control mechanism in Figs. 5, 5a, 5b, 50, again comprises aplanetary drive, of the ballbearing type, for super-imposing twomovements derived from the tuning control member. The advantage ofconstructions including planetary gearing as compared with mechanism forproviding change-over from gearing'of a given ratio to gearing of adifferent ratio, consist in seriously increased smoothness of operationat the instant of change-over from one drive ratio to the other, and inthe possibility of having lower controlpower. Use is made in the presentinstance of differential combination of the eifects of the two movementsin the planetary drive, whereby to provide more easily high values ofdrive ratios, in the low-speed condition. Slow-motion drives of thedifferential variety are of course well known; however, in the presentinstance, a few additional parts only are required which in themainprovide both the desired ratio characteristics and the controlaction. I

The inner race I2I of the ball-bearing is provided with an extensionspindle, at I06, which may directly support the tuning knob, and is thusadapted to be driven at'all times. The ballcage is directly fixed to thetuning spindle I05 or slow-motion drive thereof. The outer race isarranged to be kept stationary in the quick drive condition, by theintermediate of gearing I36- I3'II38, whereby the ball-bearing driveoperates under conventional conditions. In the slow drive condition, theouter race is driven through the spindle I06 in a sense opposite to thatof the inner race, by the intermediate of the gearing I36-I31-I38 justreferred'to, and which comprises a pinion I36 adapted to be drivenby thespindle I06 through a friction clutch, an annulus I38 with internal gearteeth, and a mediate gear wheel I31 which is freely rotating'about astationary pivot, andwhich is meshing with both the pinion I36 and outerwheel I38 to transmit the movement from the former to the latter. Theouter ball-race I23 is formed with suitable radial extension-arms forsupporting the annu-' lus I38. The toothed disk H3 is fastened to thepinion I36 and forms the driven member of the friction clutch justreferred to, which latter cornprises a flange II! on thecontrol'spindle, and a spider-shaped spring I I6. The ratio of theslowmotion drive I36-I38 should be'somewhat lower than that of theball-bearing, and may be about 1:5 if the normal ratio of the latter (i.e. assuming the outer race to be held fixed) is 1:55. In such lattercase, supposing that the inner race is held fixed, the drive ratio ofthe ball-cage with respect to the outer race establishes as the inverseof (1+1:5.5), i. e. about 55:65; accordingly, supposing that theball-cage is solely driven through the toothed wheels I36-I 3'II 38 andthrough the outer race, the inner ball-race being held fixed, the driveratio of the ball-cage with respect to the pinion I36, establishes as(1:5) (5.5:6.5) i. e. about 1:6. However, this drive condition is notactually used in the construction shown, and when the ball-cage iseffectively subject to the combined and opposing movements, which whenconsidered per se would be effective through ratios of 1:55 and of about1:6, and which both issue from the main driving spindle I06, theirdifference only will be effective in driving the ball-cage, and is aboutA the speed of movement which may take place through the first-named or"normal ratio (125.5).

The operation of the modified device in Fig. 5 is as follows: Supposethat no appreciable highfrequency energy is impressed on the controlchannel; the negative bias of the tube I9! in such event will'be low,and a substantial current will flow through the winding of theelectromagnet, which will bring about a positive arrest of the tootheddisk so as to prevent the outer ball-race I23 from being driven by thecontrol spindle, whereby the frequency-setting of the receiver will bepermitted through gearing of comparatively low ratio (such as 115.5, inthe numerical instance given in the above).

Now assume that the receiver is in resonance with a station ofsubstantial amplitude, with respect to the noise level. A substantialproportion of high-frequency energy will be impressed on the diode 46whereby to exceed the threshold bias thereof, an equivalent proportionof energy being applied to'the diode 41. Since the potentials across therespective load resistors 48 and 49 are connected in opposition; theywill cancel each other, and the outgoing line 50 will in effect be atground potential. The resulting increased negative bias of the tube I5I,which for instance may vary from -3 volts to 5.5 volts, brings about areduced anode current, reliably to cause retraction of the armature ofelectromagnet, thereby releasing the toothed disk, to permit the outerball-race I23 to be driven by the control spindle I06. The ball-cage isnow subject to the differential effects of the movements of the innerand outer ball-races, the resultant gear ratio of the latter drivingconnection in effect being about ,6 higher in the instance mentionedthan the former, whereby the tuning condensers may be rotated over thepoint of exact resonance at the normal speed, to provide an effectivespreading of the in-tune position. It will be observed that the gridpotential of the amplifier I9I does not depend on the intensity of thecarrier wave, whereby the operation of the system is in substanceindependent from the effectiveness of the A. V. C. device.

When the receiver is slightly or moderately oii resonance with respectto a carrier wave of sufficient intensity, the positive potential acrossthe load'resistor49 relatively increases, whereas the negative potentialacross the load resister 48 relatively decreases, by reason of theselectivity characteristics of the networks 45, and 43-44, these twoeffects being cumulative in lowering the potential of the outgoing line50, the variation in potential being particularly marked in proximity ofexact resonance.

" The negative bias of the tube I9I is thus considerably lowered and mayeven attain a positive value; however the resulting overload of the tubedoes not mean a serious inconvenience since in practice the tuning doesnot remain for a considerable period of time in this condition.

When the receiver is considerably oif resonance with respect to acarrier wave, no positive potential will be set up across the biasedrectifier 46; however, negative potentials will still be produced acrossthe rectifier 41. By virtue of the provision of the resonant circuit 4|,these cannot however reach a value sufficient to cause attraction of themagnet armature.

It. will now be supposed that the receiver is not tuned to a carrierwave, and that there is static of substantial intensity. Since theenergy of the static is in substance uniformly effective over a widefrequency range, (at the input of the receiver, and as transposed by thefrequency changer to extend around the intermediate frequency), andsince the feed path of the rectifier 41 is predominantly eifective at orabout the exact intermediate frequency, the potential pro: duced acrossthis rectifier, for a certain amount of energy at the input of thereceiver, will in substance be defined by the sensitivity or re sponseat the exact intermediate frequency. On the other hand, since the feedpath of the rectifier 4B is predominantly effective at frequenciesmoderately spaced from the intermediate frequency, and more particularlyat the peak response frequencies; the potential due to static, for acertain amplitude of static at the input of the receiver, willpredominantly be defined by the sensitivity or response at about thepeak frequencies, the efiect being rather analogous to that of a carrierwave moderately off resonance, as considered in the foregoing. Since thesensitivity 'or response at these peak frequencies is considerablyhigher as compared with the sensitivity at the intermediate frequency,and since the latter response is approximately the same as thepredominating response, (at intermediate frequency) across the rectifier47, there will be predominant influence of the positive output of therectifier 46. The grid bias of the relay tube 99! cannot therefore reacha sufficiently high negative bias to cause retraction of the arrestinglever. Furthermore, the excess of output of 46 is practically constantwhichever may be the intensity of the static, and same depends only onthe ratio of peak tocrevasse response, of the resulting resonance curvecharacteristic effective across the rectifier 36.

When a sufiiciently powerful station is tuned in, in the presence of amoderately intense background of static, the energy of static which inthe main is defined by the peak response frequency, will not exceed theintensity of the carrier Wave energy of the operating intermediatefrequency across the rectifier MB, and will not therefore affect theoperation of the arresting lever.

Brief interfering impulses due to atmospherics or to sparking engines orthe like, will with the same speed set up potentials across bothrectifiers, those due to the rectifier M5 being somewhat higher as inthe above instance of uniform static, and they cannot therefore cause anerroneous operation of the electro-magnet.

While I have indicated and described several selected embodiments of myinvention, it will be apparent to those skilled in the art that myinvention is by no means limited to the particular arrangements shownand described, but that many modifications may be made within the scopeof my invention, as set forth in the appended claims.

I claim:

I. A radio receiver comprising tuning means, operating means for saidtuning means, variableratio transmission mechanism coupling said tuningmeans and driving means; and means. responsive to the tuning-in of acarrier-wave to vary the transmission ratio of said mechanism.

2. A radio receiver according to claim 1, said variable-ratiotransmission mechanism comprising an electromagnetic device energized bycurrent responsive to the tuning-in of a carrierwave, saidelectromagnetic device serving to effeet the alteration in the gearingratio of the said mechanism.

3. A radio receiver comprising tuning means; operating means for saidtuning means, transmission means normally coupling said tuning means andsaid, driving means, to provide a.sub.- stantially definite normal speedof drive-of the tuning means; further transmission means adapted forcoupling said tuning means and said driving means, said furthertransmission means having a drive ratio different from that of thenormal transmission means, and serving to provide a stepwise lower speedof drive of the tun-, ing means; means responsive to the tuning-in of acarrier-wave and operable electromagnetically, for simultaneouslyrendering said normal transmission means in substance inefiective, andfor rendering effective said further transmission means.

4. A radio receiver comprising mechanism according to claim 3,characterized in that the said electro-magnetically operable meansincludes a clutch operable by current responsive to the tuning-in of thecarrier Wave, for effectively preventing the coupling of said drivingmeans and of said first-named transmission means, without simultaneouslypreventing the coupling between said driving means and said second-nameddrive transmission means.

5. A radio receiver comprising mechanism according to claim 3,additionally characterized in that the transmission path between saiddriving means and said first-named transmission means includes a clutchof the friction type, in substance positioned out of the coupling pathof the driving means with respect to the said second-named drivetransmission means, in combination with means including an electromagnetfor preventing movement of the driven clutch member, by current appliedto said electromagnet and responsive to the tuning-in of the carrierwave.

6. A radio receiver comprising mechanism according to claim 3, whereinthe means for selectively making effective in substance one drivetransmission path, includes, within the coupling path of the saidfirst-named drive transmission means, and in substance out of thetransmission path of the second-named transmission means, a frictionclutch, in combination with a locking device for selectively preventingmovement of the driven clutch member, the said locking device includinga toothed disk for said driven clutch member, and an electro-magnetoperated by current responsive to the tuning-in of the carrier-Wave, forpositively locking the said disk in position.

'7. A radio receiver comprising variable-ratio transmission mechanismaccording to claim 1,

said mechanism including planetary gearing with an output member andwith two input members, in combination with drive transmission means ofsubstantially predetermined drive ratio characteristics for coupling thedriving means and the output member through one of said input members;further drive transmission means of a different ratio for coupling thedriving means and the output member through the said second inputmember; said mechanism being further characterized in that the couplingfor one of said input members is permanently effective, in substancethereby to define a definite driving speed; whereas said other inputmember is coupled to the said driving means through a clutch; and meansfor operating the said clutch in response to the tuning-in of acarrier-wave.

8. A variable ratio mechanism for a radio tun ing device, said mechanismcomprising planetary gearing with an output member and with two inputmembers; an operating spindle for said mechanism; drive transmissionmeans 'for coupling the operating spindle and the output member, througheach of said input members, said transmission means having respectivelypredetermined but unequal drive ratio characteristics; a clutch of thefriction type in the drive transmission path between the operatingspindle and one of said input members, while out of the transmissionpath of said other input member;

a toothed disk for the driven clutch member; means including anelectromagnet operated by current responsive to the tuning-in of acarrier wave, an armature for said electromagnet; a projection for saidarmature; said projection serving positively to lock the said tootheddisk in position, in response to the operation of the electromagnet.

EGON NICOLAS MULLER.

